There are many reasons why one should believe in and follow G-d.
One is that He really exists:-).
But here is another: Without Him there is no absolute, obligatory morality. Case in point that happened recently:
A married man [who happens to be Jewish but quite assimilated although he has a smattering of Jewish knowledge] is writing a book about CHARACTER and VIRTUE. What a BEAUTIFUL topic!!!:-) Sort of like a mussar sefer for secular people. The book highlights the lives a various moral heroes throughout history and tries to analyze and understand human behavior while promoting a higher level of morality.
From a book review: The author argues [that] our society has devolved into an ever-increasing celebration of the self. Parents and schools nurture self-esteem, and value self-expression in offspring and students. The form of ambition our society celebrates is being true to oneself by pursuing one’s own passion. Broadcasting oneself through the construction of a social-media persona is nigh on compulsory; being “liked” matters even more to us than being well-liked did to Willy Loman. As a result, we have become less empathetic, and more apt to regard our relationships with others in the light of useful expediency. We are living in the “Age of the Selfie,” the author notes, adding to the more than a hundred and forty thousand uses of that phrase already tallied by Google.
At the same time, he urges, it’s not too late to join a counterculture—“to live a decent life, to build up the soul.” To help his readers do so, he’s boldly provided at the end of his book a fifteen-point “Humility Code,” which includes the assertions “We don’t live for happiness, we live for holiness” and “pride is the central vice,” as well as the injunction “No good life is possible unless it is organized around a vocation.” And yet he reassures his readers that the moral life, as he conceives it, need not require the kind of renunciation sought by St. Augustine, who is the subject of Chapter 8. “It doesn’t matter if you work on Wall Street or at a charity distributing medicine to the poor,” he writes. “The most important thing is whether you are willing to engage in moral struggle against yourself.”
The author says "I wrote it to save my own soul".
This is all a great improvement over the crass materialism and worship of pleasure and entertainment that pervades our society. Not Torah but at least an attempt to find some higher, more spiritual meaning to life.
In order to write this book the author hired a [Christian] female research assistant [young enough to be his daughter] upon whom he heaps ebullient praise in the preface. He attributes many of the ideas of the book to her and that he only had to steal from her in order to produce this work. It seems like he really appreciates and likes her.
Turns out that he REALLY does and much more than that.
He divorces his wife and marries her.
In other words - A married writes a book about morality during which he has an affair with his co-author and then divorces his wife and marries her.
He can write“We don’t live for happiness, we live for holiness" together with a woman with whom he is cheating on his wife.
So OF COURSE the Talmudic rule of אין אפוטרופוס לעריות holds eternally true [as long as we have the bodies we do]. But more than that - it was not just a temporary indiscretion but a life decision to leave his wife and children for this woman with whom he "fell in love".
While writing a book to "save his soul" about moral character and against the selfishness that is so ubiquitous in our world.
If one has no G-d then ANYTHING is possible.
I don't know all of the details of the story [nor do I want to...] and I am sure there is information I am missing but from what we know there is a big lesson here. As Dostoyevsky is often quoted as writing in his book "The Brother Karazamov" - "If there is no G-d then everything is permitted".
So we need Hashem for many reasons. One simple basic one is that there is no hope for a just society without Him.
In the words that conclude Koheles:
סוף דבר הכל נשמע את האלקים ירא ואת מצוותיו שמור כי זה כל האדם!!